1. The Field of the Invention
The present invention is related to a hybrid rocket motor case constructed of a metal shell and a fiber reinforced composite liner and methods for manufacturing such a case. More particularly, the present invention is related to an improved rocket motor case constructed of a slotted metal shell to which is bonded a plurality of sheets of continuous, unidirectional fiber reinforcement, bonded together by a thermoset or thermo-plastic matrix material and methods for manufacturing.
2. Technical Background
Historically, rocket motor cases have generally been made of metal. Metal cases have traditionally been favored because of their inherent toughness and versatility. A variety of end closures and fin attachments can be easily mounted to a metal case, making metal cases suitable for many applications. Also, metal cases can be quickly and inexpensively manufactured. Thus, metal cases have generally provided excellent performance and versatility at an acceptable cost.
However, if a rocket motor having a metal case is inadvertently ignited either through exposure to high temperatures or through bullet or fragment impact, a metal case has the potential of containing substantial pressure. Consequently, ignition pressure will escape through the rocket nozzle, generating substantial thrust, propelling the burning rocket motor or substantial pieces thereof. The dangers to crew and equipment resulting from the inadvertent ignition of rocket motors having a metal case are obvious.
As a result of these dangers, tests to determine the sensitivity of rocket motors to conditions which may cause unplanned ignition of rocket motors have been developed. Munitions successfully passing such tests are generally categorized as "Insensitive Munitions." Thus, Insensitive Munitions tests attempt to measure the sensitivity of a rocket motor to cookoff, and bullet and fragment impact.
Because the sensitivity of a rocket motor is greatly affected by the design of the motor case, various case designs have been generated in an attempt to develop a case which minimizes violence resulting from bullet and fragment impact, and mitigates reaction to both slow and fast cookoff. Unfortunately, such designs frequently result in a reduction in performance of the rocket motor.
One design which is generally recognized as providing an acceptable combination of performance and insensitivity is a filament wound composite case. Such a case is prepared by employing a high-strength, continuous reinforcing filament, such as graphite, impregnated with a graphite or epoxy resin. The case is formed by winding the filament in a predetermined pattern about a mandrel configured according to the desired interior dimensions of the case. The pattern of the winding is selected to provide the required strength in the case. The resin is then cured and the mandrel removed from the case.
By properly designing the pattern of the winding, acceptable hoop and axial strength can be obtained, providing the necessary case pressure capability in a case weighing much less than a corresponding metal case. Such composite cases perform well in Insensitive Munitions tests. In cookoff tests, thermal degradation is predictable at fairly low temperatures, depending on the resin employed in constructing the case. Bullet and fragment impact tests cause the cured composite to fracture or delaminate. Any initial pressure buildup within the case, such as that resulting from ignition of the propellant, will cause further breakage of the case thereby eliminating the possibility of any significant pressure buildup within the case.
Filament wound composite cases, however, do not provide an optimal design. In many tactical applications, the case must be attached beneath the wing of an aircraft. During flight, the case may be exposed to a variety of conditions, including substantial heat on the exterior skin of the rocket motor resulting from the friction of air passing along the case at high velocities. Such friction-generated heat is an even greater problem during flight of the rocket itself, where velocities in excess of Mach 3 can be achieved. As can be predicted by the performance of composite cases in cookoff tests, the skin of the rocket motor will begin to thermally degrade when exposed to such extreme temperatures.
Composite cases suffer from additional disadvantages in that fins, launch lugs, domes, bulkheads and other attachment devices which may form a necessary part of the case, are generally configured such that they cannot be formed in the case during the initial manufacturing process. Thus, these parts must be attached to the composite case through other means. Because of the inherent physical properties of the composite material of which the case is made, problems relating to the attachment of these parts to the composite case must be overcome.
Attachment methods successfully used for metal cases, such as welding or securing with nuts and bolts, cannot be utilized when the case is made of a composite material. Any holes drilled in the composite case to accommodate screws, for example, will form stress risers. Hence, a much greater thickness of composite must be employed to support the stresses which result from pressurization and flight loads on the case. Thus, when working with composites, the attachment of fins, end closures and other parts is generally accomplished by bonding the parts to the case with an adhesive. However, bonding with an adhesive is generally a less reliable method of attachment than those methods available for use with a metal case.
An additional concern when working with composite rocket motor cases is their lack of durability. Whereas metal cases are quite resistant to damage, composite cases are easily susceptible to breakage. Rocket motor cases may be subject to abuse resulting from a variety of sources and must be capable of withstanding substantial impact forces. As would be predicted by their performance in bullet and fragment impact tests, composite cases are easily weakened when subjected to impact forces. Thus, the fragile nature of the composite case, while positively contributing to the case's insensitivity, adversely affects its practical utility and durability.
The construction of filament wound cases requires substantial manufacturing time, the bulk of which is dedicated to winding the filament about the mandrel. The machinery to accomplish the winding is necessarily complex in order to be able to accurately lay the filament in the prescribed pattern. Additionally, the materials cost for manufacturing a composite case are higher than for a metal case. Consequently, another significant disadvantage to filament wound cases is that they are more expensive than corresponding metal cases.
Thus, it would be an advancement in the art to provide a rocket motor case which could pass Insensitive Munitions cook-off tests and could withstand the heat generated at the skin of the rocket case as a result of substantial velocities of travel.
It would be a further advancement in the art to provide a rocket motor case which could pass Insensitive Munitions bullet or fragment impact tests and still possess the durability to be damage resistant when subjected to the handling abuses and impact forces which such rocket motors typically have to endure.
It would be an additional advancement in the art to provide a rocket motor case which would be light weight, thereby facilitating the handling of the rocket motor, yet maintaining the strength required to assure acceptable performance.
It would be another advancement in the art to provide a rocket motor to which fins, end closures and other attachable parts could be easily and readily attached utilizing known, reliable attachment methods.
It would be yet a further advancement in the art to provide methods of manufacturing a rocket motor which minimize labor time and materials cost and which avoid the utilization of complex and expensive machinery.
Such an apparatus and method for manufacturing is disclosed and claimed herein.